It is hard to see the area in the photo you are referring to, however, I am assuming what you are experiencing is commonly called "Hole in the Head" or "Head or Lateral Line Erosion" (HLLE). It is not known what directly causes this but stressors can be linked to poor water quality and diet. Such stressors can lead to HLLE however it may not affect every fish. Certain species are more predisposed to catch HLLE such as surgeonfish/tangs and Angelfish. It is more similar to a condition versus disease, so it is treatable with the right steps taken.
Here is a study on HLLE by none other than
@Jay Hemdal which covers the topic quite well. It is a good source to learn a lot about the topic and possible treatments. It also has photos to view and confirm if this is indeed what your fish has. Keep in mind it ranges in severity so what your CB has could be far less severe than what is displayed in his study.
https://www.reef2reef.com/threads/head-and-lateral-line-erosion-hlle.767945/
Here is another link that showcases more examples and the different forms HLLE can display as with a few suggestions to cause of your fish's conditions. However, Jay's post will go far more in-depth.
https://www.reef2reef.com/threads/hlle-head-lateral-line-erosion.259916/